1. Field of the Invention
This invention relates to novel, improved adhesive compositions. More precisely, this invention relates to novel improved polyurethane adhesive compositions particularly useful in bonding shoe tread members.
2. Description of the Prior Art
Polyurethane compositions useful as adhesives, coatings and primers are known to the art. For example, adhesive compositions comprising polyurethane dissolved or otherwise dispersed in a suitable solvent(s) have been used for the exacting task of bonding tread members to shoes by a heat activation process. Where used here, the term "shoe" relates to outer footwear generally and includes the article in the course of manufacture. The expression "tread members" where used here relates to shoe soles, heels and sole and heel units.
Commercially available polyurethane adhesives have been used to form a satisfactory bond between shoes and tread members of various materials including roughened leather, polyvinylchloride and polyurethane. However, usually some pretreatment of the materials is required to achieve bonds of adequate strength and heat resistance for sole attaching purposes. For example, problems have been encountered in bonding tread members based on thermoplastic rubbers (e.g. styrene-butadiene-styrene block copolymers) with polyurethane adhesives, especially to shoes with polyvinyl chloride uppers. Chlorination pretreatment procedures have been employed in such instances to improve bonding of such members with polyurethane adhesives. These procedures yield generally acceptable results in many cases but precautions are required to avoid health hazards. Furthermore, the procedure frequently has to be carried out by manual operation in order to repeatedly produce consistently acceptable results. Accordingly, chlorination pretreatment is potentially hazardous or unreliable under shoemaking production conditions.
Another known pretreatment of tread members of resin rubber, i.e. high styrene-butadiene-styrene copolymer resins to improve bonding with polyurethane compositions involves the use of primer solutions which are applied to the tread member to improve adhesive performance characteristics. These primers may be applied by conventional roller coating techniques and present no greater health hazards than are created at the stage of adhesive application. However, their use is restricted to a limited number of shoemaking materials.
Attempts have also been made in the art to develop improved polyurethane compositions which can provide satisfactory bonding to a variety of surfaces so that pretreatment might be avoided or at least minimized. As those in the art know, polyurethanes employed in solution based primers and adhesives are usually thermoplastic reaction products of for example polyisocyanate, polyol and chain extender. Improved bonding of polyurethane adhesives to thermoplastic rubber has been achieved by compounding them with resins, for example polymethyl methacrylate. These resin/polyurethane compositions provide improved bond strength and heat resistance of the bonds and reduce the incidence of pretreatment required. However, the compositions have inferior properties for example in relation to spotting tack, and solution stability. "Spotting tack" is especially significant in those processes in which the adhesive is applied to the workpiece, heat activated and then the shoe and tread member are brought together with the adhesive between them and pressed together to achieve bonding. "Spotting tack" where used here means the ability of an adhesive film to adhere to a similar adhesive film when one or both are heated and they are then pressed together. A comparison of this property for various compositions is given by noting the time during which the heated film or films remain capable of adhering to an unheated adhesive film or to each other. An indication of the bond strength is given by a determination of the force required to peel a sample bond apart and the type of failure induced in the bond. An indication of heat resistance is given by noting the extent to which a bond can be separated by a given force at elevated temperature. Spotting tack in the region of 15 to 30 seconds, peel strength of more than 100 Newtons/25 mm and creep results at 60.degree. C. for 10 minutes of less than 50 mm under a peel load of 1.9 Kg are highly desirable performance characteristics for adhesives for bonding tread members to shoes.